The invention relates to the application of psychrometric charts to data centers, and more particularly relates to the application of psychrometric charts to interpretation of sensor data in data centers.
A data center is a facility used to house computer systems and associated components. The electronic components of computer systems give off heat. However, these electronic components may malfunction or be damaged in the presence of excessive heat. Further, these electronic components may be damaged by both excessive and too little humidity. In the case of excessive humidity, condensation may occur. In the case of too little humidity, the occurrence of electrostatic discharge becomes more frequent. Therefore, the physical environment of a data center is rigorously controlled. Air-conditioning is used to control both the temperature and humidity in data centers.
The American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE) has published a number of guidelines to data center environmental conditions widely used in the data center industry. These guidelines include ASHRAE Publication “Thermal guidelines for Data Centers and other Data Processing Environments”, Atlanta, 2004 (“ASHRAE 2004”) and ASHRAE Publication “Best Practices for Datacom Facility Energy Efficiency, Second Edition”, Atlanta, 2009 (“ASHRAE 2008”), each of which is incorporated by reference herein in its entirety. Each of these guidelines define an environmental envelope—a set of safe operating ranges for the data center.
Temperature and humidity have complex interrelated behavior. Psychometrics is the field of engineering concerned with behavior of mixtures of air and water vapor under varying conditions of heat. This behavior must be taken into account when both monitoring and controlling the temperature and humidity of a data center. Therefore the ASHRAE guidelines are best displayed as an envelope on a psychrometric chart to show acceptable environmental values. A psychrometric chart embodies the complex interrelation of humidity and temperature. FIG. 1 shows psychrometric chart 102.
Dry air exists when all of the contaminants and water vapor have been removed from atmospheric air. Dry air is used as the reference in psychrometrics. Moist air is a mixture of dry air and water vapor. Air temperature is a measure of the sensible heat content of air. Sensible heat is related to the changes in temperature that do not alter the moisture content of air. Latent Heat is related to level of moisture in the air. The total heat of the air, or enthalpy, includes the sensible and latent heat.
The dry bulb temperature is the air temperature determined by an ordinary thermometer. The dry bulb temperature axis 104 is located at the base of the chart. Vertical lines indicate constant dry bulb temperature.
Wet bulb temperature is the temperature reading from a wetted bulb that gives a direct indication as to the total heat content of air. It reflects the cooling effect of evaporating water. Wet bulb temperature can be determined by passing air over a thermometer that has been wrapped with a small amount of moist cloth. The cooling effect of the evaporating water causes a lower temperature compared to the dry bulb air temperature. The wet bulb temperature axis 108 is located along the curved upper left portion of the chart. The downward right sloping lines indicate equal wet bulb temperatures.
Dew point temperature is the temperature below which moisture will condense out of air. Air that is holding as much water vapor as possible is saturated or at its dew point. Water will condense on a surface that is at or below the dew point temperature of the air. The dew point temperature axis is located along the same curved portion of the chart as the wet bulb temperature axis. Horizontal lines of dew point temperature 109 indicate constant dew point temperature.
The absolute humidity or the humidity ratio is the ratio of the mass of the moisture present in the sample to the total volume of the sample. This quantity is also known as the water vapor density. The humidity ration axis 106 is located at the right of the chart. Horizontal lines would indicate equal humidity ratio.
Relative humidity is a measure of how much moisture is present compared to how much moisture the air could hold at that temperature. Lines 110 representing conditions of equal relative humidities sweep from the lower left to the upper right of the psychrometric chart.
The 100 percent relative humidity (saturation) line corresponds to the wet bulb and the dew point temperature axis line. The line for zero percent relative humidity falls along the dry bulb temperature axis line. Thus the psychrometric chart correlates five physical properties—1) Dry bulb temperature, 2) Relative humidity, 3) Wet bulb temperature, 4) Dew point temperature, and 5) absolute humidity. Knowledge of any two properties will yield the other three remaining properties. Thus the psychrometric chart embodies the relationship of these variables at a given pressure, usually sea level. These variables are also related, in some charts, to the latent and specific heat as well as the specific volume.
Sensors are typically deployed in a data center to monitor both temperature and relative humidity. The density of sensor deployment varies widely, but a sensor every 15 feet in a datacenter is not atypical. Such sensors often include both temperature and relative humidity measurements. From these two pieces of information all other psychrometric variables can be derived assuming the air pressure at sea level. Such sensor data must be evaluated against the ASHRAE envelope in order to evaluate if the datacenter is operating at a safe temperature and humidity.
Heretofore, data center operations have typically not used this sensor data in a manner that includes explicit consideration of the psychrometric variables for the equipment safety and energy-efficient operation of the data center. These considerations have recently become unavoidable. In particular, then ASHRAE 2008 guidelines define an envelope that must be considered in psychrometric context. The ASHRAE 2008 guidelines were created to allow for more energy efficient cooling of data centers relative to the old AHSRAE 2004 envelope. Where the ASHRAE 2004 envelope can be defined by a simple range of relative humidity and dry bulb temperature, the ASHRAE 2008 envelope is defined by more than two psychrometric variables and is best understood by humans on a psychrometric chart. In other words, the ASHRAE 2004 envelope was defined by 4 variables (TDRYHI, TDRYLO, RHLO, RHHI) The 2004 specifications were bounded between 20 to 25° C. (68 to 77° F.) and 40 to 55% RH. Change in the 2008 ASHRAE standards included decreasing the dry bulb lower limit to 18° C. (64.4° F.), and increasing the upper limit to 27° C. (80.6° F.) The moisture limits were lowered to a 5.5° C. (41.9° F.) dew point and increased to 60% RH & 15° C. (59° F.) dew point. In order to take advantage of ASHRAE 2008 envelope a psychrometric chart should be checked by a human operator with the environmental data against the envelope and to direct a response. It is impractical to plot such constant data checking by hand. Heretofore the industry has not taken advantage of automated methods and systems of using a psychrometric chart to check the environmental data against the ASHRAE guidelines by an operator.